1. Field of the Invention
The present invention relates to a low-cost radar, especially with high-resolution imaging. It can be applied especially to short-distance surveillance radars as such as for example radars used to monitor level crossings, cross-roads and parking lots.
Road networks still contain many level crossings, both hand-operated and automatic. Unfortunately, there are many accidents that occur regularly. To reduce the risk of accidents, it may be useful to have means to monitor level crossings. These means would, in addition, send information to approaching trains on traffic conditions in the vicinity of these level crossings. These monitoring means must be operational at all times, by day and by night. An imaging radar meets these constraints.
2. Description of the Prior Art
There are many types of imaging radars. These include especially:
Mechanical scanning radars
Electronic scanning radars,
Radars with beam-formation by computation (hereinafter called FFC)
Mechanical scanning radars cannot be used to obtain an instantaneous view of the observed scene. They are furthermore costly because of the presence of the mechanical elements needed to move the antenna. Finally, they have mediocre reliability, especially because of the wear and tear on the mechanical elements.
Electronic scanning radars for their part can be used to obtain an almost simultaneous view of the observed scene. However, they use complicated electronic circuits and have to comply with very severe manufacturing constraints, related to the control of the phase and amplitude on each radiating element of the antenna. This makes them very costly and limits their field of application.
Antennas with beam-formation by computation are used to obtain an instantaneous view of the observed scene. The control of the phase and amplitude on each of the reception antenna elements is performed directly by the algorithm for beam-formation by computation and the dispersions of the antenna and the analog reception channels may be compensated for by this processing. However, these radars use heterodyne reception. Consequently, each reception antenna element is followed by a complex reception chain comprising especially mixing, amplification, filtering and encoding means. The cost of this type of radar is therefore especially high.
It is an aim of the invention to enable the making of an economical imaging radar of the type with beam-formation by computation.
To this end, an object of the invention is a radar comprising means of beam-formation by computation associated with N homodyne reception channels, each channel giving a reception signal to the means of beam-formation by computation, the transmission antenna being positioned in such a way that its distance increases from one reception antenna to the next. The spacing between the antennas is defined so that the homodyne-related leakage is oriented in a direction located in an angular zone of the space that is not processed by the radar.
The distance from the transmission antenna, for example, increases regularly from one reception antenna to the next one. In particular, the antennas of the reception channels are, for example, aligned with a substantially constant spaced between two consecutive antennas.
Advantageously, the radar makes use of the homodyne-related leakage to carry out a self-test of the transmission/reception function.
In one embodiment, the reception antennas are, for example, made on one and the same printed circuit.
The main advantages of the invention, in particular, are that it provides high resolution on a wide frequency band, is suited to numerous applications, provides for very high manufacturing reproducibility and high operating safety, and enables modular operation of the radar.